In the electrophotographic process, in general, an electrostatic image carrier comprised of a photoconductive photoreceptor is charged and then imagewise exposed to form an electrostatic latent image thereon. The formed electrostatic latent image is then developed by a toner prepared in the particulate form by incorporating a coloring agent or the like into a binder resin. The toner image thus obtained is then transferred onto a suitable transferer such as copying paper, and is finally fixed onto the transferee, whereby a visible image is formed.
Thus, in order to obtain a visible image it is necessary to fix the toner image. For this purpose, the heat roller fixing process, which has a high thermal efficiency and is capable of rapidly fixing, has been conventionally used.
In recent years, however, the reduction of the fixing heater's power consumption to enable the fixation to be made at a lower heat roller temperature has now been strongly demanded from the necessities of (a) restraining the deterioration of copying apparatus due to overheat, (b) preventing the photoreceptor from being thermally deteriorated, (c) shortening the warming-up time required for the heat roller to be heated up to a temperature ready for fixation after the start of the fixing device's operation, (d) reducing the drop of the heat roller temperature due to the absorption of heat by copolying paper to thereby enable running copying operations to make a large number of copies in succession, (e) raising the thermal stability, and the like. Accordingly, the toner also is required to be satisfactorily fixed at a lower temperature.
Further, the toner should be present in the powdery form without adhering under the using or storage environmental condition; i.e., the toner is required to be excellent in the antiblocking property. In addition, in the heat roller fixing process, which is considered suitable for fixing, an undesirable phenomenon that a part of the toner constituting an image is transferred at the time of fixation onto the heat roller surface and the part is then retransferred onto the subsequent copying paper to stain the image thereof, the so-called offset phenomenon, tends to occur, so that the toner needs to have a capability of preventing the occurrence of the offset phenomenon; i.e., resistance to the offset phenomenon.
Thus, there have heretofore been proposals of a technique to use a polymer, as the binder resin constituting a toner, which is formed by chemically combining at least one crystalline polymerizable part having a melting point of from 45.degree. C. to 150.degree. C. with a noncrystalline polymerizable part having a glass transition point of below 0.degree. C., as disclosed in, e.g., Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 87032/1975, and a technique to use a thermoplastic polymer, as the binder resin constituting a toner, which contains in the molecule thereof a crystalline block having a melting point of from 45.degree. C. to 90.degree. C. and a noncrystalline block whose glass transition point is at least 10.degree. C. higher than the melting point of the crystalline block, and the crystalline block content of which polymer is from 70% to 95% by weight, as disclosed in Japanese Patent O.P.I. Publication No. 3446/1984.
Also Japanese Patent O.P.I. Publication No. 154740/1981 discloses a toner containing a graft copolymer comprised of a crystalline polymer formed with one or two or more monomers selected from the group consisting of ethylene, propylene and vinyl acetate, and an amorphous polymer formed with one or two or more of vinyl polymers, and further Japanese Patent O.P.I. Publication No. 8549/1982 discloses a toner containing a graft copolymer comprised of a crystalline trunk polymer part formed with at least one monomer selected from the group consisting of ethylene, propylene and vinyl acetate, an unsaturated polyester trunk polymer part, and a vinyl-type branched polymer part.
However, the toner disclosed in the foregoing Japanese Pat. No. 87032/1975, since it is a toner constituted by a copolymer formed by chemically combining a soft crystalline polymer part with an adhesive and soft noncrystalline polymer part having a glass transition point of below 0.degree. C. is disadvantageous in respect of causing a blocking phenomenon on the developing device and the like. In addition, the toner is poor in the developability because it is inferior in the triboelectrification as well as in the fluidity, thus producing a foggy, unclear image. Furthermore, the toner is so soft that it causes a filming phenomenon that it comes to adhere to the carrier grains or to the photoreceptor's surface after making a large number of copies, and then further comes to adhere to cleaning members such as the cleaning blade, thus resulting in the formed image being foggy and unclear with a low density, And this toner is caused by its softness to tend to agglomerate or to be hardly pulverized in the pulverizer when it is to be pulverized thereby at normal temperature, and thus the softness makes it impossible to obtain any desired grain size-having toner, lowers the production efficiency, and raises the production cost. Besides, this toner has a so high adhesiveness as to cause an offset phenomenon on a non-oil coated heat roller fixing device.
Also, in the technique disclosed in the foregoing Japanese Patent O.P.I. Publication No. 3446/1984, since it uses a noncrystalline block having a glass transition point as high as more than 100.degree. C., a crystalline block in an amount as large as 70 to 95% by weight must be used in order to meet the requirement of the fusibility at a low temperature, so that the nature of the soft crystalline block having a plasticity at room temperature is to be reflected upon the toner. That is, this toner is poor in the developability due to the inferior triboelectrification and fluidity because of its softness, thus producing foggy and not clear images. In addition, after a number of copying operations there occurs a filming phenomenon that the toner adheres to carrier grains or to the photoreceptor surface, and the triboelectrification becomes further deteriorated, and the toner comes to fusedly adhere to cleaning members such as the cleaning blade, thus producing foggy, unclear images with a low density. Further, in the thermally fixing method wherein fixation is performed in a short period by the application of a less oil-coated heat roller fixing device, since the above noncrystalline block's glass transition point is as high as 100.degree. C., the fixable temperature is increased, and since at the same time the amount of the crystalline block is as large as 70 to 95% by weight, the offset phenomenon tends to occur.
In addition, the toner as disclosed in the foregoing Japanese Patent O.P.I. Publication No. 154740/1981 or 8549/1982 is so poor in the fluidity that no developer in which the toner is uniformly dispersed onto carriers can be obtained, resulting in obtaining no toner having adequate triboelectrification and developability, and thus a partially skipping trouble occurs on the resulting image, making it illegible. Besides, in a number of copying operations, this toner, due to its poor fluidity, even when replenished, is not uniformly dispersed into the developer, thus producing unclear images.
As has been described above, the state of arts has been that toner which is free from these shortcomings has not yet been put into practical use.